feat(pipeline): crossfade seamless-loop arg builder (content pipeline §3.3)

tests/test_pipeline_ffmpeg_ops.py

@@ -1,4 +1,4 @@
-from tools.pipeline.ffmpeg_ops import frame_args
+from tools.pipeline.ffmpeg_ops import frame_args, crossfade_loop_args
 
 
 def test_frame_args_trims_scales_pads_and_strips_audio():
@@ -15,3 +15,21 @@ def test_frame_args_trims_scales_pads_and_strips_audio():
     assert "pad=1920:1080" in vf
     assert "format=yuv420p" in vf
     assert args[-1] == "out.mp4"
+
+
+def test_crossfade_loop_builds_head_mid_tail_xfade_concat():
+    args = crossfade_loop_args("in.mp4", "loop.mp4", duration=8.0, overlap=1.0)
+    fc = args[args.index("-filter_complex") + 1]
+    assert "trim=0:1.0" in fc                       # head = first O secs
+    assert "trim=1.0:7.0" in fc                     # mid  = [O, D-O]
+    assert "trim=7.0:8.0" in fc                     # tail = last O secs
+    assert "xfade=transition=fade:duration=1.0:offset=0" in fc
+    assert "concat=n=2:v=1" in fc
+    assert "-an" in args
+    assert args[-1] == "loop.mp4"
+
+
+def test_crossfade_loop_rejects_overlap_too_large():
+    import pytest
+    with pytest.raises(ValueError):
+        crossfade_loop_args("in.mp4", "loop.mp4", duration=4.0, overlap=2.0)